The Google File Stream app creates a virtual drive, e.g. G:\, that streams-in data from a business Google Drive account, over the internet, on-demand, without using any local disk storage, other than a small cache.
The virtual drive only exists for the interactive logged-in user session.
Magnus Box runs all backups (both interactive and scheduled backups) using a service user account. Different logon sessions have their own set of mapped drive letters, and the service session simply does not see that the virtual drive, G:\, exists.
It's still visible in the Magnus Box GUI file picker, which runs as the interactive logged-on user for performance reasons.
Ideally it would be possible to mount the virtual G:\ drive "For all users", to resolve this issue.
Currently, this does not appear to be possible in File Stream: https://productforums.google.com/forum/#!topic/apps/iRH9lD9NM90
Note: If the virtual G:\ drive can be made available to all users, the process of backing up this data requires that it be downloaded first from the Google Drive, and this may cause large amounts of inbound network traffic.
There are third-party programs for mounting Google Drive as a drive letter that may support "All Users". StableBit Cloud Drive (commercial) may do this, as well as 'rclone mount'.